1. Field of the Invention
The present invention relates to an electroluminescent display (hereinafter referred to as an "EL display") used, for example, for an indicator mounted in cars and for a display unit of information processing equipment.
2. Description of the Related Art
An EL display generally utilizes a phenomenon in which light is emitted when an electric field is applied to a phosphor such as zinc sulfide. A typical EL display is constructed by forming a luminescent element comprising an optically transparent first transparent electrode, a first insulating layer, a luminescent layer, a second insulating layer and an optically transparent second transparent electrode laminated sequentially on a glass substrate on the display side and disposing a back glass substrate above the second transparent electrode on the glass substrate to cover the luminescent element and by sealing an internal space created between the back glass substrate and the display side glass substrate (see, for example, U.S. Pat. No. 4,213,074).
However, the EL display constructed as described above may suffer from a dielectric breakdown in the luminescent element. Although sometimes the breakdown is only a small breakdown, the breakdown may be propagated starting as a small breakdown and which grows to cover the whole luminescent element. If the breakdown advances to the whole luminescent element as such, the function as the EL display will be impaired.
According to the afore-mentioned U.S. Pat. No. 4,213,074, a container containing silicon oil is stored within a vacuum chamber and a main body of an EL display in which two glass substrates are disposed facing each other is also stored in the vacuum chamber. Then, the inside of the vacuum chamber is evacuated to create a vacuum in the internal space within the EL display. Thereafter, the vacuum chamber is returned to atmospheric pressure while immersing an inlet on the EL display in silicon oil in the container. The silicon oil is injected to the internal space in the EL display by the differential air pressure at this time. The inlet is then sealed.
When the inside of the vacuum chamber is evacuated to create a vacuum in the internal space in the EL display, the two glass substrates of the EL display are deformed into a concave shape denting toward the internal space.
As used hereafter, the term "concave" when used to describe the substrates means that the substrates curve towards the internal space of the display, thereby forming a recess or depression on the external surface thereof. Similarly, "convex" means that the substrates curve outwardly and away from the internal space of the display, thereby forming a bulge on the external surface thereof.
Due to the above phenomenon, the efficiency for injecting the silicon oil drops and the two glass substrates of the EL display are maintained in a concave shape.
Once the glass substrates are deformed into such a concave shape, they attempt to return to their original shape and an inward force acts inside of the glass substrates. The same applies also to the case when the luminescent element is formed on the glass substrate and an inward force, i.e., a compression stress, acts on the luminescent element because the thickness of the luminescent element is very thin in comparison with the glass substrate. When a small dielectric breakdown occurs in the luminescent element in this state, a sectional profile of the breakdown point, i.e., a pinhole, becomes vaselike in the direction of thickness of the luminescent element and the diameter thereof on the opening side (the second electrode side) becomes smaller as compared to that on the bottom side (the first electrode side), because the compression stress, i.e., a contracting force, acts on the luminescent element. This brings about a state where the first insulating layer and second insulating layer as dielectrics do not exist between the first transparent electrode and the second transparent electrode and a current continuously flows around the pinhole, thereby advancing the breakdown. Then, when this breakdown propagates over the whole luminescent element, the functionality of the luminescent element is lost. The dielectric breakdown of the luminescent element here refers to a dielectric breakdown in general in each of the insulating layers and luminescent layer. Accordingly, it is an object of the present invention to suppress the dielectric breakdown of the luminescent element.